So-called XY tables are frequently used to position planar objects, such as wafers, in semi-conductor production devices. The planar objects lie on sliding carriages that can be moved in linear degrees of freedom X, Y and partially also in a rotary degree of freedom Rz, i.e., in all so-called in-plane degrees of freedom. X and Y denote the movement directions of the sliding carriage that are oriented perpendicularly to each other in the movement plane, and a Z axis is oriented in a direction perpendicular thereto. The rotary degree of freedom Rz of the object thus results from a possible rotary motion of the sliding carriage about the Z axis.
The position of the sliding carriage is often ascertained by grating-based optical position measuring devices, also referred to as encoders, in which a one-dimensional or two-dimensional measuring standard is optically scanned with the aid of one or more scanning head(s). To generate high-resolution position signals, such optical position measuring devices preferably employ interferential scanning principles, in which a bundle of rays emitted by a light source is split into at least two partial bundles of ray, which are superimposed interferentially after impinging upon the measuring standard once or multiple times.
When using an XY table in a machine, the object to be moved must usually be positioned in relation to a stationary tool or sensor. A tool point is thereby defined via the tool or the sensor, which is referred to as tool center point TCP, whose position relative to the object needs to be ascertained. A slight effective interspace between the measuring points of the employed position measuring devices and the TCP of the machine is an important prerequisite to ensure that guidance errors of the sliding carriage do not adversely affect the measuring accuracy in the position determination. This prerequisite is referred to as the so-called Abbe condition and states that the effective measuring point of a position measurement must be aligned with the TCP in the measuring direction. A lateral distance transversely to the measuring direction between the effective measuring point and the TCP is called the Abbe distance and should ideally be zero.
The effective measuring point of a position measuring device is sometimes also referred to as neutral pivotal point, inasmuch as tilting of the scanning head or the measuring standard of a position measuring device about the neutral pivoting point in the linear approximation does not result in a shift of the measured positional value, and thus in an error in the position determination.
If two position measuring devices or encoders using the same measuring direction are employed for the position measurement, whose scanning heads are disposed at a distance perpendicularly to the common measuring direction, then a weighted mean value generation of the two measured positional values makes it possible to shift the effective measuring point along the connecting line of both measuring points. Correspondingly, it is possible to use three stationary scanning heads, two of which detect measuring direction Y and one of which detects the measuring direction X of a sliding carriage, to arbitrarily shift the effective measuring points for the position determination in the XY plane through the three effective measuring points by linear combinations of the three measured positional values. In the following text, this XY plane is therefore also referred to as an effective measuring plane of the three scanning heads or the position measuring devices. The only prerequisite for this is that the two scanning heads having the same measuring direction are offset transversely to their measuring direction, and that all effective measuring points of the three scanning heads are arranged in a common plane parallel to the two measuring directions. Through the choice of the aforementioned linear combinations, the effective measuring points are shifted such that, in the effective measuring plane, each has a minimum Abbe distance for both measuring directions. In this manner, the effective measuring point lies at the same X and Y position as the TCP. Only an Abbe distance in the Z direction has so far been unable to be eliminated. Such a position determination using three stationary scanning heads is already known, especially in connection with two-dimensional cross-grating measuring standards. In an analogous manner, however, it is also possible that the scanning heads are jointly secured on the movable sliding carriage and scan a cross-grating measuring standard that is fixed in place.
PCT International Published Patent Application No. WO 2011/068254 describes an XY table, which includes a measuring standard, arranged as a cross grating, on the underside of the sliding carriage, which is moved by the sliding carriage and optically scanned by three stationary scanning heads mounted underneath the sliding carriage. Two of the three stationary scanning heads measure along the Y direction, as previously described, and the third scanning head measures along the X direction. This allows a precise in-plane measurement of the XY table in the effective measuring plane of the encoders. However, because of the employed scanning optics of the encoders, the effective measuring points, and thus the effective measuring plane of the encoders, lie in the plane of the cross grating measuring standard. The TCP, on the other hand, lies on the topside of the object or wafer situated on the sliding carriage and therefore has a large Abbe distance in the Z direction in relation to the effective measuring plane situated underneath. As a result, a high-precision measurement of the object position relative to the TCP is impossible. Small Rx or Ry tilting of the XY table, i.e., tilting about the X or Y axis, due to unavoidable guidance deviations, cause corresponding positional errors of the object.
The same holds true also for the devices described in European Published Patent Application No. 2 068 112, which describes a transparent XY sliding carriage, on whose topside a cross grating is applied as a measuring standard, which is optically scanned from below, through the transparent sliding carriage substrate, by three stationary scanning heads. The cross grating measuring standard is therefore arranged as a so-called rear surface grating. The object to be positioned, in the form of a wafer, lies on the topside of the transparent XY sliding carriage. In the devices proposed in European Published Patent Application No. 2 068 112, as well, the effective measuring point of the scanning heads in the Z direction therefore fails to coincide with the TCP. A precise examination of the proposed scanning reveals that given a thickness of the transparent sliding carriage substrate in the range of 30 mm to 100 mm, Abbe distances in the order of magnitude of 10 mm to 33 mm result in the Z direction. Given typical guidance variations of approximately 25 μrad with regard to the Rx and Ry tilting, measuring errors of 250 nm to 825 nm arise in the position determination, which is unacceptable for the typically high positioning specifications in applications of this type.